The present invention relates to zinc-containing, lead- and cadmium-free glass frits, a method of their production and their use. The glass frits of the invention exhibit a softening point below 510.degree. C. and are suitable for the production of decorative coating masses for glass, glass ceramics and other non-porous materials.
The customary method of producing decorative coating masses capable of being stoved, fired on, or baked, i.e., glazes, decorative colors and enamels for the decoration of glass and other ceramic and non-ceramic materials for which stoving temperatures below 750.degree. C. are required, necessitates the use of low-melting glass frits whose composition usually exhibits a high lead content. On account of the toxicity of said lead-containing glass frits for possible users and workers in the production and processing of such products, and because of environmental concerns, there is currently a rapidly growing need for lead- and cadmium-free glass frits.
A number of lead- and cadmium-free glass frits have been suggested in the literature. It turned out, however, that such products do not always meet the requirements placed on them for certain applications and are deficient in one or more properties.
Thus, U.S. Pat. No. 4,446,241 teaches a lead-free glass frit composition consisting essentially of Li.sub.2 O, B.sub.2 O.sub.3, SiO.sub.2 and of 2-33% by weight SnO.sub.2 +CaO and 2-33% by weight ZrO.sub.2 +La.sub.2 O.sub.3 ; with typical additives Al.sub.2 O.sub.3, SrO, BaO, Na.sub.2 O and F exhibiting their expected functions. The glass softening temperature (fiber softening point) of these glass frits is in a range of 535.degree. C. to 609.degree. C. The softening temperature being above 535.degree. C., as well as the Li.sub.2 O content which exerts an unfavorable influence on the mechanical qualities, limits the possibilities of use since Li.sup.+ migrates during the firing into the decorated glass, thus reducing the mechanical resistance of the decorated object. Non-toxic glass frits with a softening point below 535.degree. C., but which also exhibit a high Li.sub.2 O content (9 to 13% by weight), are also known from EP-B 0,294,502.
Glass frits known from U.S. Pat. No. 4,537,862 exhibit softening points in the same range as the previously mentioned glass frits. The composition is based essentially on B.sub.2 O.sub.3, SiO.sub.2, 10-18% by weight ZrO.sub.2 and 10-18% by weight rare-earth metal oxide; the weight ratio of ZrO.sub.2 to the rare-earth metal oxides is critical and is 1 to 1 to 1.4 to 1. Such glass frits are very expensive on account of their content of rare-earth oxides.
Glass frits in accordance with EP-A 0,347,379, with a glazing temperature around 600.degree. C., contain 38-43% by weight Bi.sub.2 O.sub.3 in addition to SiO.sub.2, B.sub.2 O.sub.3, alkali oxides and ZrO.sub.2 +TiO.sub.2. The very high Bi.sub.2 O.sub.3 content is considered by experts to be a disadvantage in regards to toxicological qualities.
A much-contested method of arriving at Pb- and Cd-free glass frits with low softening point consists in adding zinc oxide to glass frits:
For example, the glass-frit composition of U.S. Pat. No. 4,315,778 consists essentially of 30-45% SiO.sub.2, 13-18% B.sub.2 O.sub.3, 12-15% alkali oxides, 4-10% ZrO.sub.2, 18-23% ZnO and 4-10% F (data in % by weight). The glass-frit composition of EP-A 0,321,297, which absorbs UV radiation, consists of 19-45% SiO.sub.2, 20-33% B.sub.2 O.sub.3, 8-15% alkali oxides, 2-25% of one or several oxides of the series Ce, Mn, Fe, Co, Cu, V and Mo and can additionally contain, among other things, 0-35% ZnO, preferably 8-21% ZnO (data in % by weight). Other zinc-containing, Pb- and Cd-free glass-frit compositions have been described in GB-B 1,013,597 (12-13% by weight ZnO), GB-B 768,925 (4-15% by weight ZnO), JP-A 155040/81 (1-15% by weight ZnO), JP-A 186438/85 (20-25% by weight ZnO) and EP-B 0,267,154.
Obligatory components of the glass frits of EP-B 0,267,154, which soften between 450.degree. C. and 600.degree. C., are 5-14 mole % Na.sub.2 O, 8-25 mole % ZnO, 6-13 mole % B.sub.2 O.sub.3 and 45-60 mole % SiO.sub.2 ; typical additive components are K.sub.2 O, Li.sub.2 O, CaO, SiO, BaO, Bi.sub.2 O.sub.3, Al.sub.2 O.sub.3, ZrO.sub.2, TiO.sub.2 and WO.sub.3.